1. Field of the Invention
The present invention relates in general to power supply/signal conditioning modules used in connection with a resistance temperature detector (RTD); and more particularly, to a system having redundant power supply/signal conditioning modules used in combination with a single resistance temperature detector in a pressurized light water nuclear power system.
2. Description of the Related Art
A conventional power supply/signal conditioner module 10 for a resistance temperature detector (RTD) 12 is illustrated in FIG. 1. The power supplied by the module 10 is produced by a constant current source 14 connected to a .+-.15 volt power source, not shown. Since surge withstand testing is commonly performed in the control systems of nuclear power systems, a surge withstand circuit 16 is provided across the output terminals 18 and 20 of the power supply portion of module 10. A similar surge withstand circuit 22 is provided across input terminals 24 and 26 to provide protection for an input signal conditioner 28.
The conventional constant current source 14 includes a precision reference 30, such as an AD2710H manufactured by Analog Devices, which provides a constant voltage of, for example, 10.0 volts when connected to a +15 volt power supply and ground. Resistors 32 and 36 act as a voltage divider to produce a control voltage V.sub.1. An operational amplifier 38 receives the voltage V.sub.1 and a voltage V.sub.2, generated by current through a feedback resistor 40 connected between the second output terminal 20 and ground. The operational amplifier 38 is powered by the .+-.15 volt power supply and outputs a constant current to the RTD 12 via the surge withstand circuit 16 and the first output terminal 18. The constant current returns from the RTD 12 via the second output terminal 20 and surge withstand circuit 16 to flow through the feedback resistor 40, causing the voltage drop V.sub.2 by which the operational amplifier 38 is controlled.
The surge withstand circuits 16 and 22, as described above, are commonly used in control systems for nuclear power systems, but are not required by power supply/signal conditioner modules for resistance temperature detectors when surge withstand tests are not performed. An example of the surge withstand circuit 16 for the conventional power supply portion of module 10 is illustrated in FIG. 2. The circuit in FIG. 2 includes capacitors 42, 44, 46 and 48 connected across the output terminals 18 and 20. A resistor 50 and 52 is connected to each of the output terminals 18 and 20 and a fuse 54 is connected to one of the resistors, in this case resistor 50. A bipolar zener diode 56 is connected in parallel with capacitor 48. The bipolar zener diode 56 may be a Tranzorb diode manufactured by General Semiconductor Industries, Inc. The capacitors 42-48, in the surge withstand circuit 16, are typically 0.1 .mu.F capacitors except for capacitor 48 which is a 1 .mu.F capacitor.
After emerging from the surge withstand circuit 16, the constant current output from the operational amplifier 38 passes through the first output terminal 18, through the RTD 12 and back to the output terminal 20, causing a voltage drop V.sub.3 across the RTD 12. The voltage drop V.sub.3 across the RTD 12 is sensed by the input signal conditioner 28 and varies depending on the temperature of the resistor comprising the RTD 12. A typical resistance temperature detector 12 can be obtained from RdF Corporation and pressurized light water nuclear power systems typically use model number 21204. The input signal conditioner 28 as illustrated in FIG. 3 comprises an input buffer 58 and a filter 60. The input buffer 58 and filter 60 each comprise operational amplifiers 62, resistors 64 and capacitors 66.
The power supply and signal conditioner described above are usually provided as a single module which may be disconnected at cable connectors 68 (FIG. 1) for repair or replacement. However, when the module 10 is disconnected, there is no longer either a power supply or an input signal conditioner 28 connected to the RTD 12. It is possible to provide redundant input signal conditioners by simply connecting the input terminals 24 and 26 of multiple modules 10 in parallel, however there is no known system which provides redundant power supplies.